A new safe-room timer: crawlers get only 10 hours of rest per stop, refilled by killing.
The Blood Bar is a mechanic that takes effect when the party enters their tenth-floor safe room. Each crawler starts with a full bar but is now on a timer: they may spend only 10 hours in the safe room before it empties, and the bar can be refilled only by killing things. It pressures the crawlers to keep moving and killing rather than lingering safely between heats — another squeeze in the arena's escalating cruelty.
A depleting resource meter governing safe-room time.
A safe-room time limit forcing the crawlers to keep killing to rest.
Also appears in books you've already read — spoiler-safe to their endings: